1. Field of the Invention
This invention resides in the fields of multijunction photovoltaic cells and quantum dots.
2. Description of the Prior Art
Because the bandgap of colloidal quantum dots can be tuned based on their size to absorb distinct wavelengths of light [see Konstantatos, G. et al. Nature 442, 180-183 (2006); also Konstantatos, et al. Nature Photon. 1, 531-534 (2007); also Clifford, J. P. et al. Nature Nanotech. 4, 40-44 (2009); also Rauch, T. et al. Nature Photon. 3, 332-336 (2009); and also Sukhovatkin, V., et al., Science 324, 1542-1544 (2009)], colloidal quantum dots are an ideal light-absorbing material for photovoltaic devices with multiple junctions [see Sargent, E. H. Nature Photon. 3, 325-331 (2009); also Tang, J. et al. Adv. Mater. 22, 1398-1402 (2010); also Gur, I. et al. Science 31, 462-465 (2005); also Kamat, P. V. J. Phys. Chem. C 112, 18737-18753 (2008); also Luther, J. M. et al. Nano Lett. 8, 3488-3492 (2008); also Arango, A. C., et al., Nano Lett. 9, 860-863 (2008); also Choi, J. J. et al. Nano Lett. 9, 3749-3755 (2009); and also Debnath, R. et al. J. Am. Chem. Soc. 132, 5952-5953 (2010)]. Each photovoltaic junction in a colloidal-quantum-dot-based multijunction photovoltaic device can be uniquely optimized to absorb those wavelengths of light which result in the highest power-conversion efficiency. The power-conversion efficiencies of these multijunction photovoltaics can theoretically increase beyond that of single junction solar cells. As described by Sargent, E. H., in “Infrared photovoltaics made by solution processing,” Nature Photon. 3, 325-331 (2009), the theoretical power-conversion efficiency of photovoltaic devices increases when a series of a single junction photovoltaics, which each have a theoretical power-conversion efficiency of 31%, are stacked into multijunction architectures including tandem photovoltaic architectures, which have a theoretical power-conversion efficiency of 42%, and triple-junction photovoltaic architectures, which have a theoretical power-conversion efficiency of 49%.
One of the challenges in realizing the theoretical power-conversion efficiencies of multijunction photovoltaics is the high-energy barrier preventing the recombination of opposing electron and hole currents from adjacent photovoltaic junctions. In multijunction epitaxial photovoltaics, researchers reduce this high-energy barrier with an extremely thin tunnel junction of degenerately-doped p-type and n-type materials wherein the valence band on the p-side is energetically aligned with the conduction band on the n-side and the depletion region is sufficiently thin that electrons and holes can tunnel from one side of the layer to the other [See Yamaguchi, M., et al., Solar Energy 79, 78-85 (2005); and King, R. R. et al. Appl. Phys. Lett. 90, 183516 (2007)]. However, tunnel junctions are not compatible with colloidal-quantum-dot-based photovoltaics because of the tunnel junction's sequential combination of p-type and n-type materials and the processing constraints of colloidal quantum dots. Although the aforementioned high-energy barrier in organic photovoltaics has been reduced by interposing a layer of traps and metal nanoparticles between the electron transport layer and the hole transport layer [See Hiramoto, M., et al. Chem. Lett. 19, 327-330 (1990); Yakimov, A. et al. Appl. Phys. Lett. 80, 1667-1669 (2002); Kim. J. Y. et al. Science 317, 222-225 (2007)], the non-aqueous processing constraints for colloidal quantum dots and related devices preclude the implementation of the aqueous-based strategy suitable for organic photovoltaics.
What is needed in the art are compositions and methods for optimizing the recombination of electron and hole currents from adjacent photovoltaic junctions in colloidal-quantum-dot-based multijunction photovoltaic devices. Surprisingly, the present invention meets these and other needs.